1. Field of the Invention
The present invention relates to optical systems. More specifically, the present invention relates to optical devices used for separating white light into red, green, and blue light, and devices used for combining red, green, and blue light into white light.
2. Description of the Related Art
The principle of tristimulus colorimetry is used in optical imaging or display systems to generate the full range of colors. Most display systems utilize the red green blue tristimulus system in which any color is created from the appropriate combination of red, green, and blue. White light occupies part of the electromagnetic spectrum ranging from approximately 4.3xc3x971014 Hertz to 7.5xc3x971014 Hertz and within this spectrum blue and red light occupy the high and low frequency ends respectively while green represents the central frequency band of the visible spectrum.
Specific examples of display systems where the invention is applicable are Liquid Crystal Displays (LCD""s), Reflective Liquid Crystal Displays (R-LCD""s) and reflective Digital Micromirror Devices (DMD""s). These systems generally employ three such display panels one each for color red, green, and blue.
Certain display and imaging systems utilize the principle of thin film interference to separate frequency bands of white light to generate red, green, and blue. Specifically, color separation is often achieved using frequency sensitive optical multilayer coatings of dielectric materials applied to thin glass plates. The tristimulus form of color separation of white light into red, green, and blue is accomplished when these coatings are applied to a number of color selective, i.e. dichroic, mirrors. The dichroic mirrors are then arranged in any number of configurations to create channels of red, green, and blue light. However, these arrangements are bulky and require expensive support structures to support the mirrors and lenses in the desired configuration. In addition, these arrangements require lenses with long back focal lengths when used in imaging systems.
High-end projection systems require optical prisms with dichroic coatings for color management. Normally, different coating designs and a different type of prism is used for LCD, R-LCD and DMD projector types. These prisms should be as small as possible, efficient, and useable at high light flux. It would be advantageous to have a common design that could be used for both polarized and unpolarized types of projectors. An efficient prism that works well with polarized and unpolarized light needs to be made by reducing the angles of incidence on the dichroic coatings. An angle substantially less than 30 degrees is required to be an improvement over prior art.
LCD and other projection systems that require the use of single pass polarized light generally employ a x-cube (4-prism) color management component. The x-cube is very compact and minimizes the back focal length required by the projection lens. Coating design is greatly simplified when the device is used with systems using polarized light of a single type in each channel, but the large angle of incidence causes problems for systems using unpolarized light. Unpolarized light or randomly polarized light is equivalent to an equal mixture of both polarization types. A double pass arrangement as required by an R-LCD also places a difficult constraint on the coating design since light of both s-type (perpendicular to a plane of incidence) and p-type (parallel to that plane of incidence) polarization is used in each channel. A major disadvantage of the x-cube is that the nominal angle of incidence for both the red and blue dichroic coatings is 45 degrees. In general, the performance of such coatings decreases as angle of incidence increases. This leads to decreased transmission and an increased separation between the s- and p-polarized transmission curves. Another disadvantage is that these prisms are generally limited to moderate flux levels because the prisms are cemented together and will break at high flux levels.
DLP and other projection systems, which require the use of either unpolarized or mixed polarization light generally, use a Philips color prism (3-prism, double pass) for color management. These prisms are larger and more expensive than x-cubes; however, they can be used at high flux since no cemented interfaces are required. Coating design is more difficult in these cases since good performance must be obtained for s- and p-polarization simultaneously. The Phillips prism geometry is helpful since the angles of incidence on the red and blue dichroic surfaces are about 12 and 30 degrees respectively. These prisms were originally designed for color television cameras. Both camera and DLP systems were intended to be used with unpolarized light. Some problems occur when polarized light is used, particularly with the blue coating at 30 degrees angle of incidence.
Hence, a need exists in the art for an improved color management system which works well with both unpolarized light and double pass polarized light with a lower angle of incidence than has previously been achieved.
The need in the art is addressed by the present invention, a 4-prism color management device that is optimized for use with unpolarized light, that is, for both s- and p-type polarization simultaneously. It can therefore be used in both a double pass reflective LCD projection system as well as a DMD based projection system. Decreased angles of incidence will allow this device to perform better than existing devices.
In the illustrative embodiment, the invention is comprised of first, second, third, and fourth prisms. The first prism is adjacent to the second prism, the second prism is adjacent to the third prism, and the third prism is adjacent to the fourth prism. The first prism includes a first surface for inputting and outputting a beam of light. The second prism includes a first dichroic surface mounted at a first angle with respect to the first surface for reflecting light of a first color and for transmitting light of a second color and light of a third color. The third prism includes a second dichroic surface mounted at a second angle with respect to the first surface for reflecting light of the second color and for transmitting light of the third color.
In a specific implementation, the first angle is approximately 11 degrees and the second angle is approximately 20 degrees. The first, second, and third colors correspond to blue, red, and green, respectively. The first and second dichroic surfaces are optimized for both s- and p-type polarized light. The first prism can be constructed from a material different from the second, third, and fourth prisms in order to help correct lateral color aberration in the projection lens.